In dryland farming, stored soil water is an essential source of water for crop production. The amount of water available for storage and crop use in a particular soil in hillslope topography is affected by its position on the landscape. The effect of slope aspect and position on soil water and its changes throughout the year in dryland farming were studied in southeast Nebraska. North, south, and east‐facing slopes of Wymore silty clay loam (Aquic Argiudolls fine, montmorillonitic mesic) were selected for study. Four positions on each slope including summit, shoulder, backslope, and footslope were identified. Water content of the soil from 0 to 150 cm depth in 30 cm increments was measured weekly by neutron scattering for 2 years. Daily precipitation was recorded. Twenty percent more water was available in soils on the north‐facing slope than in soils on the south‐facing slope at planting and throughout the year. Soils on the east‐facing slope were the driest. They contained 50% less available water than did those on the north‐facing slope. Within slope positions, soils on the footslopes and backslopes contained an average of 4 cm more available water than soils on the summits and shoulders. Soil water sampling at planting time for crop management and yield prediction models should be done according to slope position.
Fourteen soil profiles from southwestern Indiana were studied to determine the effects of loess thickness and natural soil drainage on the profile distribution of clay and free oxides (CBD‐extractable) of Fe, Al, Mn, and Si. In all soils, Fe and Al curves follow the clay curve, suggesting that the oxides are adsorbed on clays and move with them. Natural soil drainage had little effect on profile distribution of clay, Si or Fe. Free Al2O3 contents increase slightly with poorer drainage. With thinning loess, clay and free Fe2O3 distribution changes little, free Al2O3 contents increase slightly, free MnO2 curves have more pronounced minima and maxima, and free SiO2 increases significantly in fragipan horizons. We propose that free silica moves down the profile in solution and precipitates, possibly with aluminum hydrous oxides on clays. The resulting silica or alumino silicate binds particles together to form the hard, brittle soil material of fragipans.
The effect of landscape position and slope aspect on soil water recharge under dryland farming was studied in southeast Nebraska. North, south, and east‐facing slopes of Wymore silty clay loam (fine, montmorillonitic, mesic Aquic Argiudoll) were selected. Four positions were identified on each slope, namely, summit, shoulder, backslope, and footslope. Water content of the soil from 0 to 150 cm depth in 30 cm increments were measured weekly by neutron probe for 2 years. Daily precipitation was recorded. Soils of the north‐facing slope were 10 % less efficient in recharge of available water than soils on south‐ and east‐facing slopes during all recharge periods. Soils on footslopes were 6 to 8 % more efficient than other positions throughout the year. Water storage efficiency of the soil appeared to be higher in fall than in spring. Available water recharge was 9 % more efficient during the fall than during the winter‐spring period in all soils regardless of slope aspects and positions. High correlation coefficients (r) existed when soil available water was related to rainfall in either fall or spring recharge period.
Thin loess deposits and underlying Sangamon paleosols were studied and related to modern soil development. Ninety‐one sites were sampled along four traverses extending 60–80 km eastward from the lower Wabash River, the main loess source. The ground soils, located mostly on primary divides, have a parent material sequence of silty loess (Peoria), sandy loess, upper part of Sangamon paleosol, and lower part of Sangamon paleosol. The paleosols developed in till or outwash of Illinoian age or in residuum of Pennsylvanian or Mississippian age.Soil development increases with increasing distance from the loess source. Thick loess (> 2m) soils have an A‐B2t‐C horizon sequence, whereas the thinner loess (1–2 m) soils have an A‐B2t‐(A′2)‐Bx horizon sequence. Soils with < 0.75 to 1 m of loess have B2t horizons in both loess and residuum and usually do not have fragipans. Fragipans tend to be associated with sandy loess, but usually do not coincide with it. They may be just above the sandy loess, coincident with it, or in some combination of sandy loess and adjacent parent materials. It is believed that soil moisture relations influenced by the sandy loess, or the underlying paleosols, or both promote fragipan development.
Hydraulic conductivity and seasonal patterns of water content were measured and related to soil formation in associated very poorly drained Brookston soils (Typic Argiaquolls) and somewhat poorly drained Crosby soils (Aeric Ochraqualfs) in central Indiana. In the Brookston soil the water table was near the surface for only a short time during periods of heavy rainfall in the winter and spring. The water table then stabilized near 125 cm deep, where it was controlled by tile drains. In Crosby, also tile‐drained, the water table stayed near the surface longer in the spring, then became deeper than in Brookston. The hydraulic conductivity is relatively high in the Brookston B and C and Crosby B horizons but very low in the Crosby C horizon, compact glacial till. Since downward movement is restricted in the Crosby C horizon, water tends to move laterally through the B horizon into the Brookston soil where the tile are in more permeable horizons than they are in Crosby.Available water capacities of soils can be estimated by measuring the field water contents in a profile at the beginning of the growing season and during dry periods for several years using a soil under perennial vegetation. In these two soils, however, it appears that water movement from Crosby to Brookston and the profile storage capacity are both important in determining the water‐supplying capacity of the soils.
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